The present invention generally relates to a print head used in a high-speed printer of the dot matrix type and, more particularly, to an improved composite bearing releasably attached to the front of the print head for accurately guiding the print wires toward the paper document.
The dot-matrix printing technique is characterized by allocating to each character a dot matrix having a plurality of closely spaced columns and rows. A particular character is generated by printed selected dots within the matrix.
In high-speed printers, for which the present invention is particularly well-suited, the dot matrix is delineated as a column of print wires moved along a paper document. The number of dots within a column as well as the number of columns per character are a matter of choice. One embodiment, for example, forms a character from five columns of seven dots per column, forming each character within a 5.times.7 dot matrix. At each column location, the tips of the selected print wires are driven against a paper document through an inked ribbon to generate the requisite dots, and the print head continues to move to the next column location.
The print quality is critically dependent on establishing and maintaining a precise point of impact for each wire. The impacting tips of the print wires must be closely spaced and accurately aligned along an imaginary vertical line. Any misalignment produces distortions in the printed characters since the dots then are not printed in the designated positions within the matrix.
The print head component directly responsible for properly aligning and accurately guiding the print wires is a bearing mounted on the print head in close proximity to the inked ribbon and paper document. A plurality of substantially parallel, vertically aligned and closely spaced passages extending through the bearing are adapted to slidably accommodate the print wires. Each wire fits snugly in each passage so as to experience only axial movement, though the fit is not so snug as to hinder the reciprocating movement. The wires are thus restricted, particularly because of the short travel involved, to move in the axial direction of the passages.
Aside from the accurate alignment of the bearing passages in order to precisely locate the print wires, the characteristics of a bearing should include: 1) long operating life, 2) quick and easy replaceability, and 3) low cost. A bearing must be able to maintain print wire alignment over a substantial period of usage. A satisfactory bearing should typically provide accurate print wire alignment over tens of millions of reciprocating operations. These operations occur at high acceleration rates in order to print at rates far in excess of 100 characters per second. The high rate of acceleration generates sidewise forces transmitted by the print wires to the bearing during printing. Consequently, a bearing which can maintain accurate alignment of the print wires under extended usage while being subjected to such sidewise forces is highly desirable. Also, since a bearing, like any mechanical part, has a limited life, its mode of attachment to the print head should take into consideration the need for future replacement. Therefore, its attachment should not be permanent but rather it should be easily replaceable.
Bearings available prior to the present invention have typically been made of either one of two materials, with neither being satisfactory in meeting all of the above-discussed bearing requirements. A jewel bearing, typically a ruby or a sapphire, while being satisfactorily resistant to sliding motion wearing, is liable to fracture and chip due to the above discussed sidewise forces imparted thereto by the print wires. In addition, a jewel bearing is rather expensive. To keep the cost of a jewel bearing from being even higher, only a minimum amount of machining, primarily to fashion the passages, is utilized. Consequently, the bearing contains no surface which can be used to form an easily releasable attachment. The only practical way of affixing the relatively flat and substantially rectangular shaped bearing to the print head is with a permanent attachment as, for example, with epoxy. Replacement of the bearing is, therefore, rather awkward and time-consuming.
A plastic bearing, on the other hand is relatively inexpensive and can economically be formed into a shape adapted for facilitating its replacement. However, its operating life is not as long as a jewel bearing.
Accordingly, it is the prime object of the present invention to provide an improved print wire bearing for a high-speed dot matrix printer.
Another object of the present invention is to provide a composite print wire bearing for a high speed dot matrix printer having an increased operating life.
Another object of the present invention is to provide an easily replaceable print wire bearing for a high speed dot matrix printer.
In accordance with these objects, the print head in a high-speed dot matrix printer is provided with a first somewhat compliant bearing releasably mounted to the print head housing. A "hard" element is fixedly secured to the front face of the first bearing. The aforesaid element is made of a material substantially harder than plastic, such as metal or a jewel. Guide means extending through the element are aligned with guide passages in the plastic bearing. The forward end, or nose, of the print head is provided with a pair of guide slots for receiving the plastic bearing which may be locked into position by means of a substantially U-shaped compliant brad which greatly simplifies both assembly and disassembly thereof.
In the above described bearing, the element serves as the "fine" alignment wire guide since it is adjacent the inked ribbon and the print wires follow the paths of its guideway, while the plastic block serves as a "coarse" alignment guide. Since the plastic block is relatively compliant and comprises the bulk of the bearing, it absorbs substantially all of the sidewise forces imparted to the bearing by the print wires. The useful life of the insert which experiences an insignificant amount of the sidewise forces is thereby increased with the overall effect being that of substantially increasing the useful life of the composite bearing over previous designs despite wear in the plastic block passages.